Dealing with Hydrological Uncertainty: A New Modelling Approach
Rob Swan
Manager, Water Resources Victoria
Cardno Lawson Treloar
169 Burwood Road, Hawthorn VIC 3122
AUSTRALIA
E-mail: rob.swan@cardno.com.au
Rod Watkinson
Manager, Flood Plain Mapping
Melbourne Water Corporation
100 Wellington Parade, East Melbourne VIC 3000
AUSTRALIA
E-mail: rod.watkinson@melbournewater.com.au
Martin Wong
Senior Drainage Engineer
City of Greater Dandenong
PO BOX 200, Dandenong VIC 3175
AUSTRALIA
E-mail: mwong@cgd.vic.gov.au
Abstract
Traditionally, flood modelling and assessment in Victoria has been undertaken using a combination of
various models and techniques. Usually a hydrological model, for example RORB, is used to generate
the expected flows from a design rain event which are then passed to a hydraulic model for analysis.
This approach relies on the hydrological model being calibrated or validated using a variety of lag
parameters that may not be consistent over the entire catchment area and the surface storages in the
catchment being accurately represented in the model. Two dimensional hydraulic models have often
been used for floodplain analysis in urban and rural areas. They have the advantage of including
storage areas and using tested methods to determine flow rates. If these types of models extend to
the catchment boundary, they can be used to determine the hydrological routing in the catchment as
well as the hydraulic properties by applying rainfall directly to the model’s 2D grid. This paper
examines the advantages and disadvantages of this approach utilizing an analysis of a large urban
catchment (approximately 12 square kilometres) in the City of Greater Dandenong.
1. INTRODUCTION
In the last ten years, hydraulic modelling of floodplains has advanced significantly with the introduction
of combined one and two dimensional models such as SOBEK (WL|Delft Hydraulics, 2007). Over a
similar time period, the hydrological models generally used for flood analysis, for example RORB
(Nathan, 2007) and WBNM (Boyd et al, 2007) have remained relatively unchanged, especially with
regard to the underlying assumptions used to generate flood hydrographs from design storms.
This paper examines an approach that uses the advanced capabilities of the hydraulic model SOBEK
to undertake the hydrological and hydraulic components of the study in a single model compared to a
more traditional method of generating design hydrographs in a hydrological model and importing them
for use in the hydraulic model. The two approaches have been examined and the advantages and
disadvantages of each are highlighted using information from the Edithvale, Noble Park and Parkmore
Main Drain Flood Investigation project. This project, undertaken by Cardno Lawson Treloar on behalf
Dealing with Hydrological Uncertainty
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of the City of Greater Dandenong and Melbourne Water, mapped flood extents and depths for a range
of storm events in an 11 km 2 urban catchment in Melbourne’s southeast suburbs. The project used a
combined 1D2D Sobek model to undertake the flood modelling and included all drainage infrastructure
greater than 300mm in diameter. Figure 1 shows the study area.
2. HYDROLOGICAL AND HYDRAULIC ROUTING
Hydrological modelling is concerned with two main processes; how much rainfall becomes runoff and
how that runoff is transported to the catchment outlet. The first process is a function of a variety of
parameters including soil moisture, rainfall intensity, antecedent conditions and land use amongst
other. The transport (or routing) process is dependant on catchment drainage characteristics and land
use. The method of this flow routing is what differentiates hydrological and hydraulic models.
Hydrological models generally use empirical formulae to estimate the storage and transport
characteristics of the catchment, whilst hydraulic models use physically based flow equations.
For flood modelling in urban areas, the general Australian practice is to generate rainfall and
undertake routing in a computer modelling package. The hydrographs produced may then be used as
an input to hydraulic models.
2.1.
Hydrological Routing
Hydrological modelling for flood events has traditionally been undertaken using ‘runoff-routing’
techniques in a variety of computer modelling packages. For the purposes of this paper, the term
‘runoff-routing’ is used for all models where a hydrograph is calculated by some form of routing a
rainfall excess through a representation of a catchment storage (Pilgrim D.H., 1998). Most modelling
packages generally available incorporate procedures to estimate the rainfall excess generated by a
design storm event in addition to the routing procedures used to estimate the hydrograph.
As reported in Rehman et al (2007), the simplified storage equations used to generate flow in
hydrological models mean that there is often significant variation in the results from various
hydrological models given the same input characteristics.
2.2.
Hydraulic Routing
A new way of assessing the catchment storage and flow routing can be applied using a hydraulic
model. In this method, the empirical representations of catchment storage used in runoff-routing
models are discarded and a digital terrain model (DTM) of the land surface is used. An excess rainfall
is then applied to the surface and the flow of water across the land surface is calculated by application
of the Saint Venant equations for shallow water flow. This method is also known as “rainfall on the
grid” where the grid is a mathematical representation of the land surface at a specified resolution
(Rehman et al, 2003). An example of a model grid is shown in Figure 1.
Each grid element (or cell) essentially acts as a subcatchment and the routing of flows is completed
according to hydraulic equations, where the key calibration parameter is friction. As each hydraulic
model solves similar equations, there should be little variation in routed flow results due to a change in
the model used.
The application of this method and comments on its effectiveness and limitations are contained in
Rehman et al, 2007.
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Figure 1 – Two-Dimensional Model Grid
3. URBAN FLOODPLAIN MODELLING APPROACH
In general, urban flood modelling is undertaken through the use of hydrological models to determine a
design rainfall event and peak storm flows at various locations, and a hydraulic model to simulate the
flow of water through the urban area.
Melbourne Water specifies that the RORB hydrological model (Mein, 2005) be used for all Melbourne
Water projects, which allows consistency of approach between various studies. This hydrological
model is a catchment based network model and is useful for flood estimation, design of retarding
structures and assessment of the impacts of urbanisation. As urban catchments are generally
ungauged or flows are only recorded in the piped component, these models are validated against the
Rational Method. Storage areas and flow routing based on channel type are added to the model after
the validation process.
The current best practice method of modelling urban floodplains utilises combined one and two
dimensional hydraulic models (Swan et al, 2005). This allows those elements that are of a onedimensional nature, such as drains, culverts and channels to be modelled explicitly, whilst modelling
the wider overland flows in a two-dimensional schematisation. Results from the hydrological model are
extracted and input to the hydraulic model at specified points in the drainage network. These points
are usually specified by the modeller and are based on the underground network and surface
topography. These points are part of the one-dimensional pipe elements so that as the capacity of the
pipe network is exceeded, excess flow spills into the two-dimensional model domain. This was the
method adopted to produce flood extents and depths for the example catchment.
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4. APPLICATION OF TECHNIQUES
The two techniques described above were applied to the Edithvale Main Drain catchment
(approximately 11 km 2) in the City of Greater Dandenong for the 100-year 90-minute Average
Recurrence Interval storm event. The following section examines the application of each technique
and the results obtained from the hydraulic model. In each case the same Sobek hydraulic model was
used. This model included the entire catchment area surface defined on a 5 m by 5 m rectangular grid
(of approximately 800,000 cells) and over 1400 individual underground drainage elements, comprising
all pipes greater than 300mm in diameter and some smaller pipes as appropriate. The model
topography and modeled underground drainage network is shown in Figure 2
Figure 2 - Sobek Hydraulic Model
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4.1.
Swan
RORB and Sobek Method
The flood modelling project was undertaken using RORB for the generation of the excess rainfall and
Sobek for hydraulic modelling of flows. In this case, due to the requirement to model all drainage
elements with a diameter of greater than 300mm, subcatchments are relatively small with respect to
the overall catchment size. Catchment boundaries were based on the underground drainage layout
and a total of 242 subcatchments were identified. The RORB model version 5.10 was not suitable for
the project due to a limitation on the number of hydrographs that could be produced. Cardno Lawson
Treloar, with the assistance of Melbourne Water, obtained a modified version of the program from the
developers to remedy this situation. This version became RORB Version 5.32.
Flows at the outlet of each subcatchment were extracted from the RORB model for use as inflow
boundaries to the hydraulic model. It should be noted that any flow attenuation, storage and routing
within the pipe network and overland flow areas are considered in the combined one and two
dimensional hydraulic model description. The Sobek hydraulic model calculates the speed at which
water flows based on standard hydraulic equations and is considered to give a better representation of
flow attenuation through the catchment. As such, the hydraulic model will give an accurate description
of both the surface and underground flows.
The subcatchment layout and RORB network links are shown in Figure 1.
Figure 3 – Catchment Layout and RORB Model Network
The results from the hydrological modelling were used as inputs to the hydraulic model. The
hydrologic inputs define the magnitude of total storm flow from the various subcatchments. The
overland flow is dynamically computed based on the capacity of the pipe system, once this is
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exceeded the resultant overland flow patterns are then determined from the two-dimensional hydraulic
model. The pipe system was described explicitly within the hydraulic model by pipe inverts, diameters
and manhole elevations. Due to the uncertainty of pipe conditions and allowing for some conservatism
in the analysis, a roughness coefficient (Manning’s ‘n’) of 0.015 was used for all pipes in the model.
This is higher than the typical value for concrete pipes in good condition (n=0.011, Chow, 1988) but
was considered suitable due to the age of the pipe network and allowance for additional losses due to
bends and pits.
The hydraulic roughness for the overland flow model was described using a two-dimensional
roughness map of Manning’s “n” values. This was developed by digitising different land-use zones
from the digital aerial photographs captured for the aerial survey within a GIS environment (MapInfo).
Table 4.3 summarises the land-use for determining roughness. The catchment is urbanised with large
areas of residential development interspersed with smaller commercial areas.
Table 1– Two-Dimensional Grid Roughness Classification
Land Use
Calibrated Hydraulic Roughness
(Manning’s “n”)
Car Park
0.025
Commercial
0.5
Wooded Park
0.08
Grassed Park
0.035
High Density Residential
0.2
Residential
0.15
Roads
0.018
The friction values for hydraulic modelling were based on a site inspection of the area and previous
experience with floodplain models of a similar nature.
4.2.
Hydraulic Routing Method
The same Sobek hydraulic model described in section 4.1 was used in the hydraulic routing method,
with a minor modification to remove the RORB inflows. A rainfall series was generated with the same
rainfall loss characteristics as the RORB model (10mm initial loss, Rc = 0.52, runoff from impervious
fraction 0.9), as shown in figure 4. The fraction impervious was set to the area-averaged catchment
fraction impervious of 0.57. This is a limitation of this method as the runoff coefficient cannot be varied
spatially at this time, although this will be upgraded in a future model release. It is considered that
given the relatively homogenous nature of this catchment, the impact of the non-varying impervious
fraction is limited. It should be noted that this method is presented as a proof-of-concept only.
9
Excess Rainfall (mm)
8
Excess Rainfall (mm)
7
6
5
4
3
2
1
1:30:00
1:25:00
1:20:00
1:15:00
1:10:00
1:05:00
1:00:00
0:55:00
0:50:00
0:45:00
0:40:00
0:35:00
0:30:00
0:25:00
0:20:00
0:15:00
0:10:00
0:05:00
0:00:00
0
Time
Figure 4 – Excess Rainfall (mm per period)
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Checks were undertaken to ensure that the total storm runoff volume was consistent between the two
methods. It was found that the variation between the total inflows from the RORB method and the
rainfall on grid method was less than 2%.
4.3.
Results
The modelled flood extents for each method are shown in Figure 5 below.
Figure 5 – RORB Method Flood Extent (left) and Rainfall on Grid Method Flood Extent (right).
All extents filtered to remove areas where depth <2 cm.
It can be seen from Figure 5 that the RORB method produces a flood extent that is continuous and
linked intrinsically to the location of the underground drainage network, particularly in the uppermost
reaches of the catchment. The locations of the major catchment overland flowpaths are preserved
between the two methods. It can be seen that the Rainfall on Grid method allows for significantly
greater storage on the surface of the catchment leading to reduced flood extents in downstream
flowpaths. This is due to two main factors for urban areas; firstly the smaller pipes that would collect
this surface runoff are not included in the model schematisation and secondly the friction applied to
house lots is greater than the roof friction that is important in runoff generations. This is consistent with
previous studies on this approach (Rehman et al, 2007).
The Rainfall on Grid approach does show some advantages over the traditional RORB method due to
the greater definition of catchment storage, accurate flowpath definitions for each grid cell and a faster
setup time than the traditional approach. The inclusion of the smaller pipes that were excluded from
the hydraulic model would likely increase the accuracy of the Rainfall on Grid approach.
5. DISCUSSION AND CONCLUSIONS
The results above show that it is possible to use a Rainfall on Grid approach to model urban floodplain
areas and achieve similar results to the best practice methods currently employed by floodplain
modellers. As with any modelling approach it is important to recognise when such an approach should
be used and what the relative limitations and advantages are over a more traditional method. Table 2
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indicates the advantages and disadvantages of each approach.
Table 2 – Advantages and Disadvantages of Modelling Approach
Approach
Traditional Method
(Hydrological Model and
Hydraulic Model)





Advantages
Hydrological modelling well
understood
Hydrological model may be
of use in other studies
Flowpaths are more
continuous as inflows are
less distributed
Fractions impervious can be
varied across catchment
Hydraulic model can be
simpler; less pipes and land
area required for modelling





Rainfall on Grid Approach
(full hydraulic routing)
 Entire catchment is modelled





explicitly - a flowpath is
provided for every point in
the catchment
A single model can be used,
meaning less time spent in
model development
Catchment storage areas are
accurately defined
Flows are introduced to the
model at every point
A detailed subcatchment
analysis is not required
Saint Venant equations used
for flow routing are more
accurate






Disadvantages
Hydrological model storage
and flow parameters are
empirical; may lead to
inaccurate flow estimation
Variation between different
hydrological models with
identical catchment
characteristics
Floodplain storage in
subcatchments is often not
estimated appropriately, due
to time constraints
Two models are required to
be constructed and
validated, increasing the time
to complete the project
Large complex catchment
layouts may not be suited to
the hydrological modelling
technique
Hydraulic model must cover
the entire catchment
Model runtime may be
increased
Catchment roughness and
runoff of roof areas must be
accounted for appropriately
Models may not allow
varying fraction impervious
across catchment
May require additional model
definition for 1d elements to
ensure accurate overland
flows (small pipes)
The potentially discontinuous
floodplain may lead to issues
when creating flood planning
maps
The Rainfall on the Grid approach is likely to become the new standard for floodplain modelling over
the next ten years. A thorough understanding of the application of this technique and its limitations is
essential for proper model outcomes and flood mapping results. There are some significant
advantages to the approach, especially the modelling of flows from every point in the catchment
ending the reliance on empirical relationships.
The traditional approach to floodplain modelling is certainly still valid and should not be seen as
inferior. Ultimately, it is the skill of the practitioner and their knowledge of flood behaviour that
determines the suitability of modelling outputs for their intended purpose. The hydraulic routing
approach is simply another useful tool in the flood modelling toolkit.
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6. REFERENCES
Boyd, M., Rigby, T., VanDrie, R., Schymitzek, I. (2007), WBNM2007 User Guide, University of
Wollongong, Wollongong
Mein, R. and Nathan R. (2005), RORB Model Version 5, Monash University and SKM Pty.Ltd.,
Melbourne
Pilgrim D.H., (1998), Australian Rainfall and Runoff Book Five – Estimation of Design Flood
Hydrographs, The Institution of Engineers Australia, Barton ACT
Rehman, H., Zollinger, M., and Collings, G. (2003), Hydrological Vs Hydraulic Routing
Possibilities with Two-Dimensional Hydraulic Modelling, 28th International Hydrology and Water
Resources Symposium, Wollongong, 10 - 14 November 2003
Rehman, H., Thompson, R., and Watterson, E. (2007), 47th Floodplain Management Authorities
Conference, Gunnedah, 27 February – 2 March 2007
Swan, R.;Howells, L., Bonello, D., Watkinson, R., Robertson, J. (2005), Flood Studies and Extreme
Events – Modelling, Mitigation and Assessment at Fairfield, Victoria., 4th Victorian Flood Management
Conference, Shepparton 2005.
WL|Delft Hydraulics (2007), Sobek Model Version 2.10.003, Delft, The Netherlands
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